WARSAW/LONDON (Reuters) - Prime Minister David Cameron appeared to win support from Poland on Friday for a proposed deal to keep Britain in the European Union, though an opinion poll suggested voters could still reject membership by as much as nine percentage points.

Cameron says he needs a deal to restrict EU migrant benefits, recover powers from Brussels and defend Britain against greater integration of the euro zone if he is going to be able to sell it to voters in a referendum likely in June.

But European Council President Donald Tusk's outline deal for Britain is still littered with empty square brackets. Proposals to allow Britain to delay paying benefits to workers from elsewhere in the EU are under intense scrutiny, especially from Poland, the biggest source of Britain's migrant labor force.

After meeting with Cameron in Warsaw, Jaroslaw Kaczynski, who as head of the ruling Law and Justice party is viewed as the ultimate decision maker in Poland, said he was satisfied.

"We have gained really very, very much," said Kaczynski, who is also a former prime minister and the twin brother of late president Lech Kaczynski.

"Poland has ... gained here really very much, full safety, above all, for all those who are in Britain right now, but also that those who have children in Poland will continue to receive benefits, they may be adjusted, but they will get them anyway."

Polish Prime Minister Beata Szydlo said she fully supported Cameron's proposals on improving competitiveness, removing red tape and granting proper significance to national parliaments but wanted to discuss the question of welfare benefits.

Cameron wants EU leaders to approve his deal at a summit on Feb. 18-19.

While support from Poland is crucial to any EU deal, senior officials of Britain’s 27 EU partners and the EU institutions were meeting in Brussels for a first official response from other member states to the deal on offer to Britain.

Cameron's deal may be taking shape in Europe but he faces a deeply skeptical British electorate - the campaign for Britain to leave the European Union has taken a nine-point lead over the rival "in" campaign, a poll showed.

Nineteen percent said they did not know or would not vote.

The polls, however, have faced questions about their methodology since they failed to predict Cameron's victory in the 2015 national election, and they have varied widely on the EU issue in recent months. A ComRes poll last week showed the "in" campaign held an 18-point lead over those who wanted out.

BREXIT PAINS?

A British exit would shake the Union to its core, ripping away its second largest economy and one of its top two military powers. Goldman Sachs has said sterling could fall as much as 15-20 percent if Britain votes to leave.

Pro-Europeans warn that an exit from the EU would hurt Britain's economy by excluding it from the vast European free trade zone, and could trigger the break-up of the United Kingdom by prompting another independence vote by pro-EU Scotland, while opponents of EU membership say Britain would prosper outside.

The survey for The Times newspaper, taken in the two days after Cameron set out a proposed deal, showed the biggest lead for the "out" campaign since the referendum wording was agreed last September.

The poll showed 45 percent of Britons would vote to leave the EU compared with the 36 percent who want to remain, compared with the four-point edge the "leave" campaign held last week.

Eurosceptic members of his Conservative Party said the proposals were far too weak and the British press dubbed the proposals a "farce", a "joke" or a "delusion".

"The negative press has pushed 'Leave' significantly ahead," YouGov said. "It's too early to say if the lead will persist or subside after David Cameron's crunch talks in Brussels."

German Chancellor Angela Merkel has repeatedly said she wants Britain, the second largest EU economy, to remain a member but has cautioned that Cameron must not overplay his hand.

Tusk's plan to keep Britain in the EU goes "right up to the pain threshold" of what is acceptable in Germany, said Gunther Krichbaum, a member of Merkel's conservative party and chairman of the European affairs committee in parliament.